The objectives of the proposed research are: (1) To identify the tissue, subcellular, and enzymatic sites of malathion, meperidine, and methylphenidate hydrolysis. (2) To quantify the relationships between the organism's hydrolytic capacity -- carboxyl-esterase titer -- and its biological responses to these esters. (3) To test injectable forms of carboxylerases as a treatment for malathion, meperidine, and methylphenidate poisonings. To accomplish these objectives we will (1) employ rapid and sensitive colorimetric assays for methyl and ethyl carboxylesterases, (2) survey tissue homogenates and blood serum of rats and mice for enzymatic activity, (3) prepare highly purified well-characterized carboxylesterases from rat liver and test them for activity toward the 3 selected esters, (4) relate the whole animal carboxylesterase titer to the animal's biological response to the substrate with malthion, meperidine, and methylphenidate as substrates and the use of appropriate bioassays, (5) survey the sera of various large mammals (horse, sheep, calf, goat) in search of soluble enzymes that hydrolyze malathion, meperidine, and methylphenidate. These enzymes, when located and concentrated, will be used to treat mice overdosed with substrates, in conjunction with specific antidotes for the substrates.